mm,oom_reaper: don't call mmput_async() without atomic_inc_not_zero()

Commit e2fe1456

 ("oom_reaper: close race with exiting task") reduced
frequency of needlessly selecting next OOM victim, but was calling
mmput_async() when atomic_inc_not_zero() failed.

Link: http://lkml.kernel.org/r/1464423365-5555-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>